The present invention relates to a two-output low-noise down converter circuit for reception of satellite broadcasting and, in particular, to a two-output low-noise down converter circuit which improves in the isolation characteristic of two outputs divided by switching.
FIG. 4 is a block diagram of a conventional two-output low-noise down converter circuit which is capable of outputting signals to two BS receivers by one circuit. Two signals received by an antenna 1, such as a left-handed polarization signal (hereinafter, referred to as "L signal") and a right-handed polarization signal (hereinafter, referred to as "R signal") or a horizontal polarization signal (hereinafter, referred to as "H signal") and a vertical polarization signal (hereinafter, referred to as "V signal"), are separated into two systems, and respectively fed to independent two transfer systems comprising low-noise amplifiers 2a, 2b, band-pass filters (hereinafter, referred to as "BPFs") 3a, 3b and mixer circuits 5a, 5b which convert input signal into an intermediate frequency signal using a local oscillating frequency derived from a local oscillator (LO) 4, respectively.
Then, the two signals, after converted into signals of intermediate frequency by the two independent transfer systems, respectively, are led to a switching circuit 6, where either the L signal or the R signal is selected. Although the case is the same also with the H signal and the V signal, the following description will be made with the L signal and the R signal. The two signals selected by the switching circuit 6 are amplified to intermediate frequencies by intermediate frequency amplifiers 7a, 7b, respectively, from which outputs in the following combinations are derived at independent output terminals 8a, 8b:
______________________________________ Output signal ______________________________________ Output terminal 8a L R L R Output terminal 8b R L L R ______________________________________
The signal selection of two input signals by the switching circuit 6 is judged by a comparator 9 on the basis that a voltage applied to the output terminals 8a, 8b is taken, for example, as High for 16 to 21 V DC, and Low for 10.5 to 14.5 V DC, and then the selection is effected by an output of the comparator 9. As a result, the relationship between the voltage applied to the output terminals 8a, 8b and the output signal is as follows:
______________________________________ Applied voltage Output signal ______________________________________ Output terininals 8a, 8b High .fwdarw. L signal Output terininals 8a, 8b Low .fwdarw. R signal ______________________________________
The numeral 10 in the figure denotes a power supply circuit for supplying voltage to the individual sections of the circuit.
According to the above prior art, with the aforementioned circuit construction, two signals of L and R signals, which are independent of each other, are outputted at the two output terminals. However, there are following problems caused by the circuit construction in terms of the isolation characteristic, which is one of the performances of low-noise down converters and shows, when the L signal, one of the two signals, is selected and outputted at an output terminal, the extent how the R signal, the other signal, leaks to the output terminal.
As shown in FIG. 5, the L signal or R signal which is selected and outputted after the switching circuit 6 onwards is terminated by a resistance of 75 .OMEGA. at the output terminal. The isolation characteristic of the switching circuit 6 is determined by this impedance matching, and the resulting isolation characteristic makes the isolation characteristic of the intermediate frequency circuit section of the low-noise down converter circuit, as it is.
For the two-output low-noise down converter circuit, it is not necessary to use two output terminals at the same time. When only one output terminal is used, there would occur deterioration in the isolation characteristic other than the isolation of the switching circuit unless the other output terminal is terminated by a resistance of 75 .OMEGA..
In more detail, in the conventional circuit construction, as shown in FIG. 6, with one output terminal 8b open, the signal selection by the switching circuit 6 would be such that if the DC voltage is normally Low, the R signal is normally selected at the output terminal 8b. For example, in the case where the L signal has been selected and outputted at the output terminal 8a, if the R signal is normally selected and outputted at the output terminal 8b as described above with the output terminal 8b open, then the R signal would be reflected by the output terminal 8b because of a mismatching of impedance due to the open state of the output terminal 8b, leaking to the other output terminal 8a, so that the isolation characteristic of the two-output low-noise down converter circuit would be deteriorated.
As measures for such problems, it has been practiced, for example, to attach a metal shielding plate so that the output signal reflected by one output terminal will not be connected with the other output terminal, or to attach a metal plate for grounding reinforcement, or to provide a resistor in series on the output signal line for damping of the signal, with a view to improving the isolation between the two output terminals. However, in many cases, it would be difficult to make changes in the wiring pattern required for such measures. Yet, attaching the shielding plate or the like would cause cost increases, as a further problem.